The present invention is directed to an apparatus for anchoring a tension member, such as a rock anchor where the tension member is an axially elongated steel rod formed of one or more axially extending parts with force transmission ribs extending helically around the rod and forming at least a partial thread. Further, the anchoring apparatus includes an anchoring element attachable to one end of the anchor rod and supportable against the surface of the material into which the rod is inserted. The anchoring element, generally in the form of an anchor nut, has inwardly directed projections on its inside surface with the flanks of adjacent projections forming grooves into which the ribs on the rod extend. With the apparatus secured in a borehole, when a predetermined axially extending tensile force acting on the rod is exceeded, relative movement takes place between the rod and the anchoring element until the predetermined tensile force is again established.
Rock anchors are employed in mining operations for supporting rock surfaces within an excavated area. Hot rolled steel rods with helically extending hot rolled force transmission ribs on their outer circumference are particularly suitable as tension members for such anchors. Such ribs form a partial thread on which an anchoring nut or element can be screwed when the nut is provided with a corresponding counterthread. Due to the ribs, these steel rods, which can be formed from one or more axially extending parts, afford a good bonding action in the region where they extend into a borehole and are embedded in a synthetic resin adhesive or grout and act in the manner of a ribbed reinforcing rod. At the end of the rod extending from the borehole the anchoring element can be provided by screwing a correspondingly shaped anchor nut onto the end of the rod.
In deep excavations the pressure developed by the overburden is held temporarily only by rock anchors, while the overburden constantly deforms. Accordingly, deformations in the overburden must be permitted so that a new state of equillibrium can be established in the excavation. Such deformations, however, attain a degree of magnitude which far exceeds the extensibility of steel anchor members. As a result, rock anchors with yieldable anchoring elements on the end of the rod extending from a borehole are known. In such known rock anchors, relative movement between the rod and the anchoring element can be effected when a predetermined axially extending tensile force is exceeded until the force returns to a point below the predetermined level.
In a yieldable anchoring appparatus of the above type, the anchoring element is provided with recesses or grooves suitable for receiving the force transmission ribs on the anchor rod. The force transmission ribs have flanks on the rod surface inclined relative to the rod axis. The grooves in the anchoring element have similarly arranged flanks and the anchoring element is formed so that it can expand elastically in the radial direction transversely of the axis of the anchor rod. Accordingly, when the predetermined axially extending force acting on the rod is exceeded, the anchoring element is displaceable in a sliding manner along the rod over a certain distance, note the German Patentschrift No. 31 45 923. The anchoring element can be in the form of a nut where grooves are formed in the inside surface of the nut between adjacent projections affording a thread-like arrangement.
Aside from the fact that this known anchoring element is relatively costly to produce to ensure elastic expansion, the anchoring force alternates between a maximum and a minimum value, that is, when the rod is locked in place or when the tensile force overcomes the locking action and there is relative movement with the anchoring element expanding outwardly.
In another known flexible anchoring apparatus, the anchoring element is in the form of a steel sleeve with an anchor rod extending through it. The bore in the steel sleeve converges toward an anchor plate and is filled with a material in pellet form, such as steel balls and is closed, note German Patentschrift No. 27 51 020. When a tensile force acts on the anchor rod, the filler material within the borehole is grouted within the tapering annular space. As a result, high compression occurs at locations in the pellet-form filler material or between the material and the anchor rod with plastic deformation occurring in these parts so that relative displacement occurs.
Because the position of the pellet-form material in the borehole cannot be influenced, the anchoring force and the flexibility of the anchoring apparatus is not exactly determinable. Moreover, in addition to elastic deformation, plastic deformations take place which are necessary for the anchoring effectiveness, however, can lead to the reduction in the strength of the anchor rod. Finally, rock anchors are known in which a thread is formed on the end of the anchor rod extending out of the borehole and the thread projects out from the body of the anchor rod. By means of a member slipped onto the body of the anchor rod, such as a conically perforated member, which is supported at one end against the rock and at the other end on the threads projecting outwardly from the body, a yieldability is attained when the threads are deformed or sheared off consecutively.
Apart from the fact that the anchoring element must always be placed on the anchor rod in the direction of the end in the borehole, the anchorage cannot be provided from the outside of the borehole and the required tension of the rod cannot be adjusted. Another disadvantage of this anchoring apparatus is that the end of the rod provided with the thread is drawn through the anchoring member as through a drawing die so that there is no possibility of influencing the anchoring force.